Spatial Distribution of Neutral Genetic Variation in a Wide Ranging Anadromous Clupeid, the American Shad (Alosa sapidissima)
Hasselman, Daniel James
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Species long-term persistence is to a great extent contingent on the ability of populations to mount variable responses to perturbations; the breadths of which are largely dependent on the amount of heritable variation present at the population level. However, populations are not necessarily equivalent in their amount of genetic variation, or in their responses to future environmental conditions, and information about the magnitude and spatial distribution of intraspecific genetic variation is integral to conservation planning, and preserving species evolutionary potential. Using neutral molecular markers, I demonstrate that latitude is an important determinant of the amount and spatial distribution of genetic variation within and among Nearctic fishes. Latitudinal declines observed among species were mirrored by declines within species, and encourage a cautionary approach for interspecific comparisons and inferences of broad spatial patterns of genetic variation when data for individual species are obtained from only a portion of their range. I subsequently examined the magnitude and spatial distribution of microsatellite based variation for 33 spawning runs from across the range of American shad (Alosa sapidissima). Sequential reductions of intraspecific genetic variation with latitude were observed among spawning runs from formerly glaciated regions; consistent with stepwise post-glacial range expansion, and successive population founder events. Canadian populations exhibited temporally stable genetic differentiation characterized by a significant pattern of isolation by distance, and exhibited evidence of metapopulation structure. Although isolation by distance was observed among U.S. spawning runs, population structure was comparatively weak; a possible consequence of increased levels of gene flow (human-mediated or natural). Different spatial patterns of population structure were detected across the species’ range, but are not likely due to alternative reproductive strategies (iteroparity vs. semelparity). Rather, these different patterns probably reflect different management strategies in Canada (no stocking) and the United States (stocking), alternative glacial histories, or combinations thereof. Reciprocal patterns of genetic diversity and differentiation observed across the species’ range suggests that U.S spawning runs contribute more to diversity and less to differentiation than Canadian spawning runs. These results have implications for future shad restoration efforts, and the long-term persistence of the species.